1. Field of the Invention
This invention relates to a composite, thermally insulated architectural frame member. More particularly this invention is concerned with metal windows, doors and the like having an exterior metal frame sections and an interior metal frame section which are thermally insulated from each other by a thermal break section between the exterior and interior frame sections.
2. Description of the Prior Art
Metal building units such windows and doors are widely used in construction and the use is increasing because of the consistent performance, low maintenance and excellent durability.
In the manufacture of the metal building units there are a number of important factors to be considered. The first is the ease of manufacture. Metal windows and doors to a large extent are manufactured on a custom or semi-custom basic due to the wide range of size combinations and types of units required. For example it is not uncommon for a building such as a home to have many different size windows from large picture windows to small bathroom windows, and the types of windows can be large single pane picture windows, double hung windows and casement windows. It is according important that the manufacturing method be readily adapted to make different sizes and types of building units.
Another important factor to be considered with respect to metal building units is the thermal performance in use as compared to wooden frame windows. Wood has many problems such high maintenance and limited durability, but wood is inherently a very good thermal insulator and therefore prevents thermal transfer through the frame. Most metal building units are made with extruded aluminum frames. Aluminum, and other metals, unlike wood, are poor thermal insulators. Accordingly the problem of transfer of heat and resulting "sweating" of metal frames must be taken into consideration.
A further important factor to be considered with respect to metal building units is short term and long term mechanical stability. The metal building units such as windows are assembled in a factory with glass panels held in place by the metal frames. The assembled units must be able to be shipped to the job site and installed without coming apart. Once a window is installed the frame holds the glass in place during use. Considerable positive and negative force is applied to the installed windows panes by wind on the exterior side. The force applied to the window panes is transferred to and carried by the window frame. The window frames must be sufficiently strong so as not to come apart and allow the window panes to become loose or fall out. The metal frames must maintain their structural integrity over the useful life of the metal building unit which can be many years.
When metal buildings units such as windows were first introduced the frames were made with one piece metal extrusions which formed the interior and exterior portions of the frames. The single piece frames had adequate strength properties but were found to be unsatisfactory because of heat transfer though the frame and moisture and frost forming on the frames. It was suggested to make the frames with separate inner and outer sections to limit the thermal transfer. The multipart units were unsatisfactory as they were much more difficult assemble and install properly as the inner and outer portions had to be separately installed.
To overcome the problems of the separated multi-part units it was suggested to connect the exterior and interior sections with a section called a "thermal break" made of a material having low heat conductivity such as low heat conductive plastic. The thermal break has the dual function of thermally isolating the inner section from the outer section of the frame and holding the sections together so the building unit can be installed as a unit.
The methods which were hereto suggested for forming the thermal break were far from satisfactory. The methods were complex and time consuming resulting in a substantial increase in production time and cost. One such method is disclosed in Hetman, U.S. Pat. No. 3,099,337, where is it suggested to force an oversized strip of a thermal insulating material endwise into one section of the metal frame using the end of the section of a metal frame to shape the strip to size and then forcing a second section of the metal frame into a groove formed in the thermally insulating material. This method was unsatisfactory because it was found difficult to end wise force feed the sections together, particularly if the sections were relative long. In addition the assembled frames did not have the required mechanical strength as the sections rotated in use and tended release the sealing pressure on the window panes.
A further suggestion to make metal frames having an inner and outer sections separated with a thermal break was disclosed in Burbank, U.S. Pat. No. 4,704,839 and in Rawling, U.S. Pat. No. 5,187,876. According to these patents a one piece extruded metal section is formed with a pocket in the center web. A curable liquid resin, such as a polyurethane, is then inserted in the pocket and allowed to cure. After the resin has cured an elongated strip of metal is machined away to separate the starting single section into two separate sections, thermally isolated, but secured together by the cured resin.
In Hetman, U.S. Pat. No. 4,067,163 there is disclosed an improved method for forming a molded in place thermal break. The improvement disclosed by Hetman '163 is to apply a release agent before forming the cast in place thermal block to inhibit the adhesive bonding of the thermal break and the metal parts and thereby that avoid destructive forces in the thermal break section due to differences in thermal expansion and contraction of the inner and outer metal sections.
The methods of forming the thermal breaks in place as suggested by Burbank, Rawling and Hetman described above all have certain inherent problems. The casting of the liquid resin in place is a costly and time-consuming process. The required accurate mixing of the components of resin presents problems in commercial production. Also the requirement of post machining the starting metal part into two sections adds to the manufacturing cost. The most serious problem, however, encountered with the above described prior art methods is that the cast in place thermal breaks have been found to mechanically and chemically break down in a relatively short period of time. Since it is the thermal break which holds the exterior and the interior metal sections together, when the thermal break fails, the window fails, literally falling apart and resulting in a very dangerous condition.
The exact reason for the failure of the materials used for the cast in place thermal breaks is not known. It is believed however that the cast-in-place polyurethanes typically used in this application do not have adequate long term properties to withstand the conditions encountered in extended exposure to the elements. The exterior side of a window can reach temperatures as high as 180 degrees Fahrenheit with dark colored exterior frames in the summer and as low as 30 to 40 below zero Fahrenheit in the winter. If the thermal break is bonded to the metal portions the difference in the expansion and contraction of the inside portions of the window, the outside portion of the window and the thermal break can mechanically fracture the resin. A further problem is that the cast-in-place resins in general and the polyurethane resins in particular do not have the required long term chemical stability due to thermal cycling and hydrolysis.
The problem of failure of the thermal breaks is a long standing, well known, problem in the art. The suggestions heretofore made to correct this problem have at best been stop gap measures or compromises. Coulston, U.S. Pat. No. 4,377,926 (1983) clearly recognized the problem of failure of the thermal breaks and suggested using metal sections, one of which has a pocket, the other of which has a tee shaped extension which is positioned in a locking relationship in the pocket of the other metal part. The resinous thermal break is cast in place in the pocket to separate the metal parts. If the resinous thermal break fails the metal parts will become loose but will be held together by the tee shaped extension in engagement with the pocket to provide what is referred to as a "fail safe" window. This suggestion does not however correct the underlying problem of failure of the thermal breaks and presents problems in holding tolerances in manufacturing.
An alternate suggestion to make a "fail safe window" was disclosed in Meigs et al., U.S. Pat. No. 4.423,578 (1984). In Meigs et al. was suggested to remove only portions of the metal strip between the inner and outer sections rather than completely removing the metal strip as noted above. This method is referred to as "skip-debridge." The remaining metal portions in the strip hold the inner and outer metal sections together if the thermal break fails. This however is at best a compromise in that it partially defeats the purpose of having a thermal break as heat and cooling are transferred through the remaining metal portions. In addition this method has been found to increase buckling of the metal parts as a result of heat transfer across the remaining metal portions in the separation strip.
In Hetman, U.S. Pat. No. 3,289,377, it was suggested to use an extruded thermal plastic strip as the thermal break. According to Hetman '377 the strip was first molded to shape and then post formed by stretching to reduce it size. The strip was then inserted in the frame and had to be heated to return the strip to its original shape. In practice the method was found to be too cumbersome for commercial production and the results were not dependable.
The problem of assembling windows having thermal breaks to prevent heat transfer through the metal window frames and providing rigid assemble is also addressed in the foreign prior but the problem still exists. For example, in Swiss patent number 420 567 issued Sep. 15, 1964 a method is disclosed wherein one metal section is formed with a dove tail shaped extension which fits into a mating grove in a thermal break section. The thermal break section in turn is fitted into a second metal section. The specific method used employs a second extruded metal section that as formed has a wall which is bent out of position to allow the thermal break section to be installed in the open channel of the second section. After the assembly is completed, the bent out wall is forced back into position to hold the thermal break in place. The method in the Swiss is unsatisfactory in commercial production in that post forming of the metal section is difficult because it is deep within the assembled window and requires a separate step in the assemble process. Also, the required roll forming of the bent out wall causes the extrusion to bow, making it unstable. Furthermore, in order to securely hold the thermal break in position, the thermal break section has to be some what resilient rather than rigid which detracts from the mechanical stability of the assembled window. The disclosed dovetail likewise is insufficient in width which further adds to the instability of the final window.
What would be highly desirable would be a metal building unit such as a window having an exterior metal portion, an interior metal portion and a thermal break which is easy and inexpensive to assemble, has an effective thermal break, can have different finishes on the interior and exterior metal surfaces and which has short term and long term mechanical and chemical stability.